Partial oxidation employing a complex oxide of transition metals as a catalyst is industrially very important, because it produces intermediates of high value for chemical engineering such as aldehydes, organic acids, epoxy compounds, nitrile compounds, and the like.
In partial oxidation, complete oxidation that produces carbon dioxide and water is inhibited, and it is important how selectively a desired partial oxidation product is produced. Thus, a catalyst used in partial oxidation should be able to inhibit the generation of carbon monoxide or carbon dioxide, and simultaneously, it should have high activity so that it may be economically valuable. Generally, partial oxidation employs a metal, a single metal oxide, a complex metal oxide, or the like as a catalyst, and particularly a complex metal oxide is predominantly employed.
Since partial oxidation is generally an exothermic reaction, and selectivity of the partial oxidation is sensitive to reaction temperature, a complex oxide catalyst that shows activity even at a low temperature is required. Catalysts for partial oxidation are widely employed in various fields.
In general, among the complex oxide catalysts, acrylic acid and methacrylic acid are prepared by gas phase contact oxidation of acrolein and methacrolein that are produced by partial oxidation of propylene and iso-butylene. This preparation process is carried out under as low an oxygen concentration and as low a reaction temperature as possible, so that combustion due to an increase in oxygen concentration of reactants is avoided, overreaction is prevented, and selectivity of acrolein or methacrolein is not deteriorated. Additionally, a catalyst that shows high activity at a low reaction temperature has higher inactivation resistance than a catalyst showing comparatively low activity. Therefore, many studies regarding a catalyst for partial oxidation of propylene and iso-butylene have been undertaken.
Thus far, in order to effectively prepare acrylic acid or methacrylic acid by gas-phase contact oxidation of acrolein or methacrolein, various methods have been suggested.
As examples, Japanese Laid-Open Patent Publication Sho 44-12129 discloses a catalyst consisting of molybdenum, vanadium, and tungsten; Japanese Laid-Open Patent Publication Sho 49-11371 discloses a catalyst consisting of molybdenum, vanadium, copper, tungsten, and chromium; Japanese Laid-Open Patent Publication Sho 50-25914 discloses a catalyst consisting of molybdenum and vanadium; and Japanese Laid-Open Patent Publication Sho 52-85091 discloses a catalyst consisting of one or more components selected from the group consisting of molybdenum, vanadium, copper, antimony, and germanium.
Also, European Patent No. 023,859 discloses that acrolein conversion rate and acrylic acid yield are varied according to methods for molding a catalyst when the components and compositional ratio of a catalyst is identical, and also discloses a process for preparing a catalyst having high acrylic acid yield. In addition, Korean Patent Application No. 1998-073605 discloses a process for preparing a catalyst suspension by controlling the weight of water based on total weight of metal salts, and Korean Patent Application No. 1998-073604 discloses that catalyst activities are varied according to particle sizes.
However, these methods have problems in that a lot of time and cost are required for drying due to the use of a large amount of water when preparing a catalyst, and the control of catalyst properties is difficult.
For improvement of the drying method, Japanese Patent No. 2002-316047 discloses a method using a microwave when preparing a catalyst for oxidation of propane. However, because this method uses water when preparing a catalyst for oxidation of propylene and iso-butylene, a phase separation occurs due to precipitation of metal components when preparing a catalyst slurry. If a drying process using a microwave is used for a layer-separated slurry, catalyst activity is deteriorated. The present invention employs an organic acid such as nitric acid and citric acid instead of the layer-separated slurry to prepare a completely dissolved slurry, and improves catalyst activity by drying the thus-prepared slurry using a microwave.
[Technical Problem]
In order to solve the problems of the prior art, it is an object of the present invention to provide a process for preparing a catalyst for partial oxidation of propylene and iso-butylene, which can reduce time and cost required for drying and improve physical properties of the catalyst.
It is another object of the present invention to provide a catalyst for partial oxidation of propylene and iso-butylene, which shows high activity for conversion of propylene and iso-butylene, shows high selectivity for partial oxidation, and can obtain acrolein and methacrolein with a high yield.
[Technical Solution]
In order to achieve these objects, the present invention provide a process for preparing a catalyst for partial oxidation of propylene and iso-butylene represented by the following Chemical Formula 1, which process comprises the steps of:                a) dissolving a metal salt comprising        i) a molybdenum salt,        ii) a bismuth salt,        iii) an iron salt,        iv) one or more kinds of salts of metals selected from the group consisting of cobalt, tungsten, vanadium, antimony, and nickel, and        v) one or more kinds of salts of metals selected from the group consisting of potassium, rubidium, and cesium,        in a nitric acid solution or in an organic acid solution to prepare a catalyst suspension;        b) drying the catalyst suspension of step a) in a microwave oven;        c) pulverizing and molding the dried catalyst of step b); and        d) calcining the catalyst powder obtained in step c)MoaBibFecXdYeOf  [Chemical Formula 1]        (wherein X is cobalt, tungsten, vanadium, antimony, or nickel,        Y is potassium, rubidium, or cesium,        each of a, b, c, d, and e represents the atomic mole ratio of each metal, and when a is 12, b is 0.5˜2, c is 0.5˜2, d is 3˜8, and e is 0.005˜0.2,        and f is determined according to the oxidation state of each metal.)        
The present invention also provides a catalyst for partial oxidation of propylene and isobutylene represented by the above Chemical Formula 1, which is prepared by dissolving a metal salt comprising a molybdenum salt; a bismuth salt; an iron salt; one or more kinds of salts of metals selected from the group consisting of cobalt, tungsten, vanadium, antimony, and nickel; one or more kinds of salts of metals selected from the group consisting of potassium, rubidium, and cesium in a nitric acid aqueous solution or in an organic acid solution to prepare a catalyst suspension, drying the catalyst suspension in a microwave oven, pulverizing the dried catalyst, and calcining the pulverized catalyst powder.
[Advantageous Effects]
According to the present invention, time and cost required for drying when preparing a catalyst can be reduced, physical properties of a catalyst can be improved, and a catalyst for partial oxidation of propylene and isobutylene, which shows higher propylene and isobutylene conversion rates and high selectivity for partial oxidation, and can obtain acrolein and methacrolein with a high yield, can be stably prepared.